In recent years, in the field of core networks, a technique of realizing high-speed transmission that exceeds several hundred Gbit/s or 1 Tbit/s for each wavelength has been studied. This technique includes, for example, optical OFDM (Optical Orthogonal Frequency Division Multiplexing), SuperChannel, and the like.
In order to realize a high-speed transmission such as the above, an optical transmitter performs a parallel operation of generating an optical signal from a plurality of transmission signals as illustrated in, for example, FIG. 1A. In this example, the optical transmitter illustrated in FIG. 1A includes high-frequency circuits 201a and 201b, an optical modulator 202, and a light source 203.
Transmission signals A and B are input to the high-frequency circuits 201a and 201b, respectively. Transmission signals A and B are electric signals, and are generated from transmission data strings. The high-frequency circuits 201a and 201b include, for example, an amplifier, a filter, and a wiring pattern. The wiring pattern is a conductor pattern formed on a printed circuit board to propagate electric signals. The high-frequency circuits 201a and 201b respectively generate, from transmission signals A and B, driving signals for driving the optical modulator 202. The optical modulator 202 modulates a continuous wave generated by the light source 203 with the driving signals to generate an optical signal.
As described above, the optical transmitter illustrated in FIG. 1A generates an optical signal from a plurality of transmission signals, and transmits the optical signal. Although an optical signal is generated from two transmission signals in the example of FIG. 1A, an optical transmitter that generates an optical signal from more transmission signals (or more electric signals) has been implemented in practical use. Configurations using a greater number of transmission signals are capable of achieving higher transmission rate.
Meanwhile, the demand for downsizing of an optical transmission equipment has further increased in recent years. Accordingly, downsizing and/or integration is important for an optical transmitter (or an optical transceiver module that includes an optical transmitter) that is installed in the optical transmission equipment.
However, downsizing of an optical transmitter causes an increase in electromagnetic crosstalk in the optical transmitter. In the optical transmitter illustrated in FIG. 1A for example, the distance between the high-frequency circuits 201a and 201b is reduced, and crosstalk between the high-frequency circuits 201a and 201b increases. In other words, there is a possibility that downsizing of an optical transmitter will cause deterioration of transmission performance. Further, electromagnetic crosstalk may occur not only between the high-frequency circuits 201a and 201b but also in the optical modulator 202. For example, electromagnetic crosstalk may occur between electrodes for transmitting high-frequency signals in the optical modulator 202.
The above described crosstalk is suppressed by shielding the high-frequency circuits 201a and 201b electromagnetically by using shields 204a and 204b, respectively, as illustrated in, for example, FIG. 1B. However, configurations of using electromagnetic shields for suppressing crosstalk prevent the downsizing of optical transmitters. Also, it may be difficult to electromagnetically shield crosstalk in the optical modulator 202.
As described above, downsizing of an optical transmitter that generates an optical signal from a plurality of transmission signals increases crosstalk between the transmission signals in the optical transmitter. And, increases in crosstalk in an optical transmitter may deteriorate the transmission performance.
As a related art, a technique of compensating for skew that occurs on the path of each channel is proposed (Japanese Laid-open Patent Publication No. 2010-193204, for example).